This invention relates to control units for externally commutated converters in general and more particularly to an improved control unit which includes means for smoothing the synchronizing voltage provided in the control unit.
Control units for externally commutated converters are well known in the art. For example, see the book "Line-Commutated Converts" by G. Moltgen, published by Siemens AG and Pitman Publishing 1972, Chapter 8, particularly pages 301 and 307 and the book "Thyristor Phase-Control Converters and Cyclo Converters" by B. R. Pelly [Wily Interscience, 1971].Aslo see DIN, 41,750 sheet 7, page 4. In these prior art control units, synchronization is a problem. In many cases they operate based on a zero crossing of a reference voltage, which in many cases is the line voltage. If harmonics are present on the line voltage, detection of proper zero crossing becomes difficult. In some cases the zero crossing can be shifted by several degrees. In very unfavorable cases a plurality of zero crossing can occur one after the other particularly where there are commutation dips in the line voltage caused by other converters. Thus, the operation of control units of this nature is quite unsatisfactory particularly in power supply systems with variable frequency and a heavy distortion of the line voltage waveshape. To remedy this situation, the use of R-C, L-C or bandpass filters is known. Such filters are described in the above mentioned references for use in smoothing the synchronization voltage. However, these filters have high cost, the chokes being quite expensive. In addition, they take up a great deal of space and are not always completely effective. Furthermore, they lead to phase shifts in the case of a frequency change so they cannot be used in networks of different frequency. In other words, the system with a filter designed for 50 Hz cannot be used in a 60 Hz system without readjustment or replacement of parts.
In view of these difficulties, the need for an improved method of smoothing a synchronizing voltage for use in a control unit for an externally controlled converter which is primarily frequency independent becomes evident.